Stanford University is recognized as a leader in the development and study of radiation therapy, from the first use of the medical linear accelerator in 1955 to the understanding of the influence of tumor hypoxia on therapeutic response in the 1970's and 1980's to the development of intensity-modulated radiation therapy in the 1990's to the ongoing development of image-guided radiation therapy. However, Stanford lacks a system for delivering radiotherapy to small animals with the conformality and image guidance routinely achieved in the clinic. We therefore request funds to purchase a Small Animal Radiation Research Platform (SARRP) from XStrahl. The SARRP is a compact irradiator employing a 225 kVp x-ray beam mounted on a rotating frame, capable of delivering beams from multiple angles and incorporating collimators to shape the beam. In addition, the SARRP can acquire and reconstruct cone beam x-ray computed tomography data, facilitating the use of volumetric imaging to guide radiation therapy. This system will be employed by a cohesive set of core users spanning multiple departments (Radiation Oncology, Radiology, Pediatrics, Medicine) and research interests (tumor and normal tissue radiobiology, stem cells, and technology development). The PI has a track record in the development of small animal radiotherapy systems and has assembled a team to deploy and manage this device within the Stanford research environment. With this new technology we will significantly enhance the translational potential of NIH funded research and will develop new science and technology.